Test: Module 01 — Introduction to Kotlin¶

Instructions: Answer all questions. Write your answers directly below each question. Bonus questions are optional and can raise your score above 100%.

Total Points: 37 pts (Easy: 5 + Medium: 10 + Hard: 12 + Expert: 10) Bonus Available: 5 pts

Section 1: Easy Questions (1 pt each)¶

Q1. What does the ? suffix on a type (e.g., String?) signify in Kotlin?

Q2. What is the difference between val and var in Kotlin?

Q3. What year did Google announce first-class Kotlin support for Android, and at which event?

Q4. What operator would you use to provide a default value when a nullable expression evaluates to null?

Q5. True or false: Kotlin requires semicolons at the end of statements. Explain.

Section 2: Medium Questions (2 pts each)¶

Q6. Explain what a "platform type" (String!) is in Kotlin and in what situation it appears. Why is it potentially dangerous?

Q7. What is the difference between these two declarations? When would you choose each?

val list = listOf(1, 2, 3)
val mList = mutableListOf(1, 2, 3)

Q8. Explain why Kotlin code can call Java libraries without any special bridge or wrapper layer.

Q9. What is a single-expression function in Kotlin? Rewrite the following as a single-expression function:

fun square(n: Int): Int {
    return n * n
}

Q10. Compare Kotlin's when expression to Java's switch statement. List at least three differences.

Section 3: Hard Questions (3 pts each)¶

Q11. Find and fix the bug(s) in this code:

fun getLength(text: String?): Int {
    return text!!.trim().length
}

fun main() {
    println(getLength(null))     // should print 0
    println(getLength("  hi  ")) // should print 2
}

Write the corrected version and explain what was wrong.

Q12. Write a Kotlin function safeDiv(a: Int, b: Int): Int? that: - Returns a / b when b is not zero - Returns null when b is zero

Then write a function printDivision(a: Int, b: Int) that calls safeDiv and prints either the result or "Division by zero" using a single expression (no if statement — use the Elvis operator or let).

Q13. Given this sealed class hierarchy:

sealed class Shape
data class Circle(val radius: Double) : Shape()
data class Rectangle(val width: Double, val height: Double) : Shape()
data class Triangle(val base: Double, val height: Double) : Shape()

Write a function area(shape: Shape): Double that computes the area of each shape. Use a when expression and make it exhaustive (no else branch). Then explain why removing is Triangle -> ... would cause a compile error.

Q14. Write a Kotlin function buildUserSummary(name: String?, age: Int?, city: String?): String that returns a string summarizing a user, gracefully handling all combinations of null parameters. Examples: - All present: "Alice, 30, from Prague" - Name only: "Alice" - Name and city: "Alice, from Prague" - All null: "Anonymous user"

Section 4: Expert Questions (5 pts each)¶

Q15. A Java library's method is declared as:

public class UserService {
    @Nullable
    public String findEmail(long userId) { /* ... */ }
}

A Kotlin developer writes:

fun notifyUser(userId: Long) {
    val email = userService.findEmail(userId)
    sendEmail(email.uppercase())   // crashes in production
}

Explain precisely why this crashes, what Kotlin's platform type system has to do with it, and rewrite notifyUser so it handles null correctly with appropriate logging.

Q16. Design a Result<T> type from scratch in Kotlin. It must: 1. Be a sealed class with two subclasses: Ok<T> (holds a value) and Err (holds an exception) 2. Have an extension function Result<T>.getOrDefault(default: T): T 3. Have an extension function Result<T>.map(transform: (T) -> R): Result<R> that applies the transform only on success 4. Have a top-level function fun <T> runCatching(block: () -> T): Result<T> that wraps a block and catches exceptions

Show all declarations and provide a usage example demonstrating all four.

Bonus Questions (variable pts)¶

Bonus 1. (+5 pts) Explain the concept of "smart casts" in Kotlin with a code example. What are the conditions under which a smart cast does NOT work? Give at least two examples of situations where smart cast fails and explain why.